RAPID COMMUNICATION

Japanese Journal of Applied Physics 51 (2012) 080202
http://dx.doi.org/10.1143/JJAP.51.080202

High-Speed Measurement of Refractive Index Using Dielectric Multilayer Films
Deposited on Optical Fiber End
Kyung-Su Kim, Yosuke Mizuno, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Received May 8, 2012; accepted June 3, 2012; published online July 12, 2012
We have recently developed a refractive index (RI) sensor using a band-pass filter composed of dielectric multilayer films deposited on an opticalfiber end. The RI is detected as a shift in the wavelength characteristics, and can be read with an optical spectrum analyzer used in this sensor. In
this study, we employ a high-speed demodulator, which converts the optical wavelength information into electrical voltage through the slope of thespectral characteristics. A response time of as short as several tens of microseconds is achieved. We also compare the RI sensitivity of theproposed sensor with that of a simple Fresnel-based RI sensor. # 2012 The Japan Society of Applied Physics

Refractive index (RI) sensors have been used ina wide range of applications including biochemistry, chemical/environmental analysis, and ultrasonic-wave detection.1–15) Many types of RI sensor using
optical ﬁbers have been developed so far because of theirfollowing advantages: compact size, cost eﬃciency, ﬂexibility in design, remote-sensing capability, multiplexibility,and immunity to electromagnetic interference. Not only
intensity-domain techniques that exploit the RI dependenceof the Fresnel power but also a variety of frequency-domaintechniques have been developed. Typical examples include
RI sensors based on (1) ﬁber Bragg gratings (FBGs),1–4) (2)long-period gratings (LPGs),5–8) and (3) interferometers.9–12)
Although these techniques have their own advantages, noneof them is applicable to the RI measurement of solids.
To extend their application range, we have developed anovel RI sensor using a band-pass ﬁlter (BPF) comprisingdielectric multilayer thin ﬁlms (DMFs) deposited on an
optical ﬁber end.16) This structure, called a BPF on a ﬁberend (BOF),17) eliminates incident light with a particular
wavelength, which is dependent on the RI of the materials
attached. The BOF-based RI sensor has many advantages
such as high spatial resolution, cost eﬃciency, simplicity,robustness, and applicability not only to gases and liquids
but also to solids. Its measurement speed was, however, lowowing to the use of an optical spectrum analyzer (OSA) witha second-order sweep time, which needs to be enhanced forsuch applications as real-time RI monitoring13) and ultrasonic-wave detection.14,15)
In this study, we demonstrate high-speed RI measurementwith the BOF-based sensor by using a fast demodulator,
which converts the optical wavelength information into
electrical voltage using the linear slopes of the reﬂection
spectra. A several tens of microseconds response time isexperimentally achieved, which corresponds to the dynamic
behavior of the sample liquid. The RI sensitivity of the BOF
sensor is also compared with that of a simple RI sensor
based on Fresnel reﬂection to clarify its beneﬁts.
For high-speed RI measurement, the optical wavelength
information needs to be converted into electrical voltage.
This conversion is made up of two steps: from wavelength
information to optical power, and from optical power information to voltage. To achieve the ﬁrst step, a linear slope
region of the reﬂection spectra needs to be selected,17–20) towhich the wavelength of a light source is set. The second
step can be easily achieved by using a photodetector (PD),which operates much faster than the OSA.

Fig. 1. (Color online) Schematic experimental setup for high-speed RI
measurement. DAQ, data acquisition; ISO, isolator; OSC, oscilloscope; PC,personal computer; PD, photodetector; TLD, tunable laser diode.

The experimental setup for the RI measurements basedon high-speed demodulation without comprising an OSA isschematically shown in Fig. 1. The structure of the BOF
was the same as that previously reported.16) Judging fromthe reﬂection spectra of this BOF,16) the wavelength of thetunable laser diode (TLD) was set to 1543.5 nm. Its output
light with a power of 7 dBm was injected into the BOF via
an optical circulator. The reﬂected light from the BOF wasconverted to an electrical signal using a PD with a bandwidthof over 5 MHz, and was monitored employing an oscilloscope(OSC) with a sampling rate of 250 MS/s. As materials to bemeasured, we employed air and ﬁve types of liquid: water(n  1:33), ethanol (n  1:36), index-matching oil (I) (n 1:44), matching oil (II) (n  1:60), and matching oil (III)(n  1:74). The BOF was immersed into the liquids by hand.
Figure 2 shows the measured temporal change ofthe output voltage for the ﬁve types of liquid, which wereattached to the BOF in air. The voltage decreased according
to the RIs of the diﬀerent liquids. The time constant of thePD was estimated to be less than 0.2 s, since the cutoﬀ
frequency was over 5 MHz. However, a several tens of
microseconds response time was observed for the voltagechange, which was much longer than the time constant of thePD. This delay may be attributed to the time for the sample
liquids to cover the sensing area of the BOF, which is determined by their density, viscosity, and surface tension. FromFig. 2, the RI dependence of the output voltage can beplotted as shown in Fig. 3. The slope of its linear approximation was 27:95 mV/RIU (refractive index unit). The
slight discrepancy of the experimental data from the approximated line is probably caused by the fact that, as RIchanges, the optical power also slightly changes.16)
Next, we compared the experimental sensitivity of theBOF-based RI sensor with the theoretical sensitivity of a080202-1# 2012 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 51 (2012) 080202

K.-S. Kim et al.

Fig. 5. (Color online) Comparison of RI dependences of sensitivity
between BOF and F-RI sensors.

Fig. 2. (Color online) Temporal change of the output voltage.

reﬂection spectra. The response time of the sensor is subject
to the dynamic behavior of the sample liquids, and was asshort as several tens of microseconds. The RI sensitivity ofthe BOF sensor was also compared with that of a simple F-RIsensor, and the BOF sensor was, with ease of handling, foundto be more suitable for the measurement of RI of approximately 1.46. The next task to be performed is to identify theoptimal BOF structure for maximizing the RI sensitivity. We
believe that this information will be of great signiﬁcance inapplying the BOF sensor to real-time RI monitoring as wellas to ultrasonic-wave detection in the future.

Fig. 3. (Color online) RI dependence of the output voltage.

Acknowledgements The authors are indebted to Dr. Onoda and Mr.
Nakano, Watanabe Co., Ltd., for providing the BOF samples. One of the authors(YM) is grateful to the Research Fellowships for Young Scientists from the Japan
Society for the Promotion of Science (JSPS) and to research grants from theFoundation of Ando Laboratry and the Murata Science Foundation.

1) W. Liang, Y. Huang, Y. Xu, R. K. Lee, and A. Yariv: Appl. Phys. Lett. 86(2005) 151122.
2) A. Iadicicco, A. Cusano, A. Cutolo, R. Bernini, and M. Giordano: IEEE

Photonics Technol. Lett. 16 (2004) 1149.

3) X. Shu, B. A. L. Gwandu, Y. Liu, L. Zhang, and I. Bennion: Opt. Lett. 26(2001) 774.
4) X. Sang, C. Yu, T. Mayteevarunyoo, K. Wang, Q. Zhang, and P. L. Chu:Sens. Actuators B 120 (2007) 754.

5) V. Bhatia and A. M. Vengsarkar: Opt. Lett. 21 (1996) 692.
6) H. Tsuda and K. Urabe: Sensors 9 (2009) 4559.
7) J. H. Chong, P. Shum, H. Haryono, A. Yohana, M. K. Rao, C. Lu, and Y.

Fig. 4. (Color online) Comparison of RI dependences of reﬂectivity

between BOF and F-RI sensors.

Zhu: Opt. Commun. 229 (2004) 65.

8) H. J. Patrick, A. D. Kersey, and F. Bucholtz: J. Lightwave Technol. 16

simple Fresnel-reﬂection-based RI (F-RI) sensor composedof a silica single-mode ﬁber (SMF) end. A comparison of theRI dependences of the reﬂectivity between the two methods
is shown in Fig. 4, where we normalized the vertical axis,i.e., the reﬂectivity, so that it becomes 100% when thesensing ﬁber ends are open in the air. The RI sensitivity canbe evaluated using the absolute values of their slopes, whichare given in Fig. 5. While the sensitivity of the F-RI sensor
drastically changes according to RI, that of the BOF sensor
is constant regardless of RI, which leads to ease of handling.
Within the RI range of approximately 1.46, the sensitivityof the BOF sensor is higher than that of the F-RI sensor. It
has been shown that this range can be further extended by
optimizing the BOF structure.21)
In conclusion, high-speed RI measurement was successfully demonstrated with the BOF-based RI sensor by fastdemodulation, where the optical wavelength shift was converted into electrical voltage change using the slopes of the(1998) 1606.
9) Z. Tian, S. S.-H. Yam, and H.-P. Loock: Opt. Lett. 33 (2008) 1105.
10) T. Wei, Y. Han, Y. Li, H.-L. Tsai, and H. Xiao: Opt. Express 16 (2008)

5764.
11) Y. Wang, M. Yang, D. N. Wang, S. Liu, and P. Lu: J. Opt. Soc. Am. B 27

(2010) 370.
12) Z. L. Ran, Y. J. Rao, W. J. Liu, X. Liao, and K. S. Chiang: Opt. Express 16(2008) 2252.
13) J. L. Elster, M. E. Jones, M. K. Evans, S. M. Lenahan, C. A. Boyce, W.

Velander, and R. VanTassel: Proc. SPIE 3911 (2000) 105.

14) J. Staudenraus and W. Eisenmenger: Ultrasonics 31 (1993) 267.
15) B. Shen, Y. Wada, D. Koyama, R. Isago, Y. Mizuno, and K. Nakamura:

Proc. SPIE 7753 (2011) 77539W.
16) K. S. Kim, Y. Mizuno, M. Nakano, S. Onoda, and K. Nakamura: IEEE

Photonics Technol. Lett. 23 (2011) 1472.
17) S. Onoda, N. Tsukamoto, M. Ogino, K. Yamashita, O. Yumoto, K. Inoue,and Y. Komatsu: IEEE Photonics Technol. Lett. 20 (2008) 688.

18) Y. J. Rao: Meas. Sci. Technol. 8 (1997) 355.
19) D. C. Betz, G. Thursby, B. Culshaw, and W. J. Staszewski: Smart Mater.

Struct. 12 (2003) 122.

20) J. R. Lee and H. Tsuda: Meas. Sci. Technol. 17 (2006) 2920.
21) K. S. Kim, Y. Mizuno, M. Nakano, S. Onoda, and K. Nakamura: IEICE

080202-2

Tech. Rep. 111 (2011) 43.

# 2012 The Japan Society of Applied Physics